[PERF-001] Inefficient Roadmap Query Causes Timeouts

[AndrewHakmi]

Problem

Roadmap query uses unbounded graph traversal causing timeouts on large graphs.

File: backend/app/api/engine.py (lines 187-209)

Current query:

MATCH (sub:Subject {uid: $su})
MATCH (sub)-[:CONTAINS*]->(t:Topic)  -- No depth limit!
OPTIONAL MATCH path = shortestPath((t)-[:PREREQ*..3]-(f:Topic {uid: $focus}))
RETURN t
LIMIT 50  -- Applied after full scan

Impact

• Query timeouts on large subject trees
• High memory usage
• Slow response times (>10s)
• Poor user experience

Fix

MATCH (sub:Subject {uid: $su})
MATCH (sub)-[:CONTAINS*1..5]->(t:Topic)  -- Add depth limit
WITH t LIMIT 100  -- Limit early
OPTIONAL MATCH path = shortestPath((t)-[:PREREQ*..3]-(f:Topic {uid: $focus}))
RETURN t
LIMIT 50

Additional optimizations:

• Create index: CREATE INDEX topic_subject_idx FOR (t:Topic) ON (t.subject_uid)
• Add query timeout: Configure dbms.transaction.timeout
• Profile query: Use PROFILE to verify improvements

Testing

# Profile before fix
PROFILE MATCH (sub:Subject {uid: $su})...

# Check execution time and db hits
# Should be <500ms

Estimated Time

4-5 hours (including testing)

Files

• backend/app/api/engine.py:187-209

References

• COMPREHENSIVE_TODO.md: PERF-001